In recent years, lithium batteries are in wide use as batteries of high energy density and high voltage. These batteries comprise a positive electrode of manganese dioxide or graphite fluoride, and a negative electrode of lithium and afford a high voltage of at least 3 V. However, they are primary batteries and are not chargeable.
On the other hand, lithium secondary batteries which can be charged are dischargeable many times at a high energy density and high voltage and therefore have many advantages. Although active research has been made on such batteries in many fields, those which are fully serviceable still remain to be developed. For the secondary batteries under development, transition metal chalcogen compounds such as MoS.sub.2 and TiS.sub.2, and oxides such as MnO.sub.2, Cr.sub.3 O.sub.8 and V.sub.2 O.sub.5 are used as positive electrode active substances, and lithium, lithium-aluminum alloy, etc. as negative electrode active substances. For use in such batteries, electrolytic solutions are proposed which are prepared by dissolving an electrolyte such as LiClO.sub.4 or LiBF.sub.4 in propylene carbonate, 1,2-dimethoxyethane or like aprotic organic solvent. The secondary batteries heretofore developed are still unsatisfactory because of the following problems.
The first of the problems is attributable to the organic solvents used for electrolytic solutions. Many of the aprotic solvents widely used at present are low in boiling point and inflammable, are liable to stain neighboring parts and cause ignition or inflammation due to spillage or a break and have a likelihood of explosion due to an error in use or overcurrent. Further repeated discharge and charge electrolytically form on the negative electrode a dendritic deposit of metallic lithium, which gradually grows to eventually cause short-circuiting on reaching the positive electrode.
The second problem arises from the use of the chalcogen compound, oxide or like crystalline substance as the positive electrode substance. Generally with crystalline substances, penetration of Li ions into the crystal lattice due to discharge collapses the crystal structure, rendering the substance unable to retain the original characteristics as crystals. This phenomenon occurs especially markedly in the case of deep discharge involving penetration of a large quantity of Li ions, such that if deep discharge is repeated several times, the battery becomes unserviceable owing to a great decrease in its capacity.
To solve the first of these problems, Sequlir et al. disclose a novel battery comprising a thin film of solvent-free polymer electrolyte (Extended Abstracts, 163rd Meeting, Electrochemical Society, 1983, 83, 751 Abstract, No. 493). The literature, however, merely mentions that the electrolyte is found usable at a medium temperature of about 100.degree. C. when tested, indicating that it is not usable at room temperature. P. M. Blonsky et al. further state that polyphosphazene is useful as an electrolyte for use in electrochemical batteries (J. Am. Chem. Soc., 106, 6854, 1984). Nevertheless, they merely show data as to a.c. conductivity in the range of 30.degree. C. to 97.degree. C. and fail to realize charge and discharge with d.c.
To overcome the second problem, JP-A-116758/1986 proposes the use as the positive electrode active substance of an amorphous substance prepared by adding P.sub.2 O.sub.5 to V.sub.2 O.sub.5 and melting and quenching the mixture. However, the battery thus proposed is not completely free of the second problem, with the first problem still remaining unsolved.
Japanese Patent Application No. 1028/1987 discloses a battery wherein an oligoethyleneoxypolyphosphazene is used as the electrolyte and a layer of V.sub.2 O.sub.5 as the positive electrode active substance to overcome the first and second problems, whereas the disclosed battery has yet to be improved in resistance to excessive discharging and in charge-discharge characteristics.
An object of the present invention is to provide an all solidstate secondary battery substantially free of formation of dendrite and spillage, having no iginitability because of the features of flame retardancy and low vapor pressure, excellent in safety against explosion, rupture or the like and also in resistance to excessive discharging and having a prolonged charge-discharge cycle life.